Java Packaging Guidelines

These guidelines are laid out in order of relevance to packaging.

Introduction

Background

Traditionally, Java implementations have been available under a non-free license. Free software clean room implementations of the class library largely centred around GNU Classpath. GCJ, a Java frontend for GCC, allowed for native compilation of Java software. In 2007, Sun released its reference implementation of Java under the GPL+Classpath exception as OpenJDK. This included the bytecode interpreter, just-in-time (JIT) compiler (Hotspot), and the majority of its class library. Due to the remaining small proprietary encumbrances, a project known as IcedTea was formed to build OpenJDK with entirely free tools, and provides Free software plugs for the encumbered pieces of the class libraries. Recent (early 2008) developments have enabled Fedora to ship a package under the OpenJDK name.

The Basics

The term Java means many things to many people: a class library, a bytecode interpreter, a JIT compiler, a language specification, etc. For the vast majority of users and developers, Java is a programming language and runtime environment that is architecture- and OS-agnostic. The normal flow of code is .java (source file)  .class (Java bytecode)  .jar (a zip archive). In the majority of cases, a user executes a Java program by specifying a class name containing a main method (just like C and C++). Often, this is done by invoking the java binary with a list of JAR files specifying the classpath like so:

java [-cp <jar1:jar2:jar3>] <main-class> [<args>]

Java Packaging

The JPackage Project has defined standard file system locations and conventions for use in Java packages. Many distributions have inherited these conventions and in the vast majority of cases, Fedora follows them verbatim. We include relevant sections of the JPackage guidelines here but caution that the canonical document will always reside upstream: JPackage Guidelines . Over time, we would like to remove any divergences in these documents, but where they are different, these Fedora guidelines will take precedence for Fedora packages.

Package naming

Packages MUST follow the standard Fedora Packaging/NamingGuidelines . Java API documentation MUST be placed into a sub-package called %{name}-javadoc.

Release tags

For now, refer to the Packaging/JPackagePolicy for release tags. That document should eventually be folded into this one.

Jar file naming

If a package provides a single JAR file it must have the same name as the package itself.

ex. jaf.jar

If the project name and the commonly used JAR filename differ, a symbolic link with the usual name must also be provided.

ex. Single JAR complete naming. Project name is jaf, common name is activation.

activation.jar  jaf.jar

If the package provides several JAR files, the filenames assigned by the build must be used. Above symlinking rules apply.

ex.

ant-1.5.3.jar
ant-optional-1.5.3.jar

If the number of provided JAR files exceeds two, you must place them into a sub-directory.

If a project offers the choice of packaging it as a single monolithic jar or several ones, the split packaging should be preferred.

Directory structure

All JAR files MUST go into %{_javadir}. Exceptions include JNI-using JAR files , and application-specific JAR files (ie. JAR files that can only reasonably be used as part of an application and therefore constitute application-private data).

Java API documentation uses a system known as javadoc. All javadocs MUST be installed into %{_javadocdir}.

For detailed instructions on the JPackage/Fedora maven, see the JPackage Maven rpm readme located here .

Packaging JAR files that use JNI

Applicability

Java programs that wish to make calls into native libraries do so via the Java Native Interface (JNI). A Java package uses JNI if it contains a .so

Note that GCJ packages contain .sos in %{_libdir}/gcj/%{name} but they are not JNI .sos.

Guideline

JAR files that require JNI shared objects MUST be installed in %{_libdir}/%{name}. The JNI shared objects themselves must also be installed in %{_libdir}/%{name}. If the JNI-using code calls System.loadLibrary you'll have to patch it to use System.load, passing it the full path to the dynamic shared object. If the package installs a wrapper script you'll need to manually add %{_libdir}/%{name}/<jar filename> to CLASSPATH. If you are depending on a JNI-using JAR file, you'll need to add it manually -- build-classpath will not find it.

Rationale

This is less convenient, but cleaner from a packaging point-of-view, than putting the JAR file in %{_javadir}, and putting the JNI shared object in %{_libdir} to be loaded from the default library path. First, JNI shared objects are dlopen'd, and dlopen'd shared objects should not be placed directly in %{_libdir} since they are application-private data, and not libraries meant to be linked to directly -- that is, not meant to be shared. Second, placing the JAR file in %{_javadir} causes the build-classpath script to always load it, even when running on a runtime environment of the wrong arch, meaning that the System.loadLibrary line would fail.

The plan is to eventually eliminate patching of the System.loadLibrary line and wrapper script by making jpackage-utils multilib aware. This involves the following changes: creating %{_libdir}/java and %{_libdir}/jni directories; giving JNI-containing packages the ability to require an architecture-specific runtime environment; adding support for specifying the required runtime architecture in a wrapper script; modifying jpackage-utils's runtime scripts to search %{_libdir}/java; modifying IcedTea to look for JNI shared objects in %{_libdir}/jni.

The %{_jnidir} rpm macro defines the main JNI jar repository. Like %{_javadir} it is declined in -ext and -x.y.z variants. It follows exactly the same rules as the %{_javadir}-derived tree structure, except that it hosts JAR files that use JNI.

%{_jnidir} usually expands into /usr/lib/java.

Things to avoid

Pre-built JAR files / Other bundled software

Many Java projects re-ship their dependencies in their own releases. This is unacceptable in Fedora. All packages MUST be built from source and MUST enumerate their dependencies with Requires. They MUST NOT build against or re-ship the pre-included JAR files but instead symlink out to the JAR files provided by dependencies. There may arise rare cases that an upstream project is distributing JAR files that are actually not re-distributable
by Fedora. In this situation, the JAR files themselves should not be redistributed -- even in the source zip. A modified source zip should be created with some sort of modifier in the name (ex. -CLEAN) along with instructions for reproducing. It is a good idea to have something similar to the following at the end of %prep (courtesy David Walluck):